1. Field of the Invention
The invention relates to a precise paper feeding device in a printer.
2. Description of Related Art
There is a conventional printer using a serial scan printing method by which a reciprocatable printing head installed on a carriage prints a printing line during each scan of the carriage. With this kind of printer, after the printing head prints the printing line, the carriage immediately returns to the start of the line. The moment the carriage returns, a predetermined amount of paper is fed.
Conventionally, in this kind of a printer, the paper is fed by rotating a pair of paper feeding rollers. The paper is sandwiched between a pair of paper feeding rollers, installed on the upstream and downstream sides of the paper feeding direction of the printing head, and rotatable pinch rollers in a predetermined pressure contact with each of the feeding rollers. In this case, since the feeding power of the paper is determined by the frictional force of the paper and the paper feeding rollers, the paper feeding rollers are generally comprised of an elastic member such as rubber which has a high coefficient of friction with a paper.
However, as a predetermined amount of a paper must be fed for each printing line to properly space the lines of printed characters or Figures, when the paper feeding amount is not accurate, a second printing line is to close to or partially overlayed onto the preceding printing line or extra space is created between the printing line and the preceding printing line. The result is an unattractive print job.
Because the paper feeding accuracy is important, particularly in printing pictures or Figures, a paper feeding accuracy of tens of microns might be demanded. However, it was difficult to produce a paper feeding roller, made from an elastic member like rubber, with high accuracy in order to achieve the desired precise paper feeding.
If the opposing pressure of the pinch rollers is increased to reduce the paper feeding amount, and the frictional force of the paper becomes stronger, the paper feeding rollers are distorted because the paper feeding rollers are made from an elastic member. As a result, the paper feeding accuracy becomes unstable. On the contrary, if the opposing pressure of the pinch rollers is weakened to increase the paper feeding amount, and the frictional force of the paper becomes weaker, insufficient frictional force is obtained and the paper feeding accuracy again becomes unstable.
Further, if the paper feeding amount of the downstream side paper feeding rollers is less than that of the upstream side paper feeding rollers, the paper becomes loose between the downstream side paper feeding rollers and the upstream side paper feeding rollers because of the difference between the paper feeding amount of the downstream side paper feeding rollers and that of the upstream side paper feeding rollers.
In addition, the printing speed of the printing head is adjusted to the paper feeding amount of the upstream side paper feeding rollers. When the trailing end of the sheet of paper leaves the upstream side paper feeding the rollers, that is, the upstream side paper feeding rollers finish feeding the paper, the paper feeding amount is insufficient for the printing speed of the printing head because the paper is fed only by the downstream side paper feeding rollers which have the lower amount of paper feeding.
In order to solve these problems associated with the paper feeding amount, the downstream side paper feeding rollers can be made a little larger than that of the upstream side paper feeding rollers. To make the paper feeding amount of the downstream side paper feeding rollers larger than that of the upstream side paper feeding rollers, the diameter of the downstream side paper feeding roller should become longer than that of the upstream side paper feeding rollers. However, since the difference of the diameter of the two rollers must be extremely small, production is difficult and costly. Thus, it is not desirable to process or manufacture the rollers to such close tolerances.